This project is designed to provide evidence needed for risk assessment of human exposure to methylmercury from seafood, i.e. whether or not a neurotoxic risk is present and, if so, the threshold for such effects. As the unborn child is known to be particularly sensitive, a cohort of 1000 children has been assembled in the Faroe Islands where extremely high methylmercury exposures occur. Intrauterine exposures were determined by mercury analysis of umbilical cord blood and maternal hair collected at consecutive births during 21 months in 1986-1987. A tentative limit of 10 ppm for mercury in maternal hair has been proposed by IPCS to protect against prenatal toxicity; this limit was exceeded in 13% of the births, and 25 % of the cord blood samples had a mercury concentration above the corresponding limit of 40 micrograms/l. However, no definite case of methylmercury poisoning has been observed so far. The persistence of mercury in the body will be assessed from hair-mercury concentrations of the children at ages 1 year and again at 6 years, and dietary information will also be collected. As neurotoxicity is the critical effect and may not become apparent before the preschool or school age, all children will be examined at age 6 years. A detailed pediatric examination will be carried out, and a test battery will identify possible subtle signs of neurobehavioral dysfunction. The test battery will include psychological tests and neurophysiological measurement of evoked potentials; these methods are known from previous research to be particularly sensitive to the types of neurotoxicity expected. The results of this study will also provide information on the sensitivity and feasibility of such a test battery in a large-scale population study. The Faroese population is particularly well suited for such a study because of the homogeneity and stability of the population and the efficient coverage of the Nordic-type health care system. The cohort includes 75% of all births occurring during the sampling period. No major selection bias of the cohort has been identified,in relation to pregnancy outcome. A high participation rate (about 80%) is expected at the examination of the 6-year-old children. Alcohol use is minimal in Faroese women (75% were abstainers during pregnancy), and 60% are non- smokers. The lead exposure is low (median lead concentration in cord blood, 1.7 micrograms/100 ml). Due to the high seafood intake, selenium exposure is increased, and its possible protective action against mercury toxicity will be examined.